In November 2009 the United States Preventive Services Task Force updated its 2002 national guidelines for screening mammography. They transitioned from advising women to obtain a routine screening mammogram annually starting at the age of 40 to recommending a routine screening mammogram every two years starting at the age of 50. Although this change ignited a national debate, there was consensus on the need for a decision-making resource to help women in their 40s understand the issues so they could make an informed choice in the matter. In response, we designed, built, and tested a web-based breast cancer screening decision aid tool for women ages 38 to 48 with an average risk of developing breast cancer. The tool gave these women the resources they needed to work with their healthcare providers to make choices about screening mammography that were right for them. Engineered and developed following the International Patient Decision Aid Standards criteria, the tool was refined through three rounds of usability testing and ongoing feedback from select stakeholders.